1. Field of the Invention
The present invention relates to a vehicle waste heat recovery device, and more particularly to a vehicle waste heat recovery device employing a Rankine cycle.
2. Description of the Related Art
In response to calls from society for reductions in the amount of discharged carbon dioxide (CO2), there is a need for improvements in the fuel efficiency of vehicles having an engine, such as automobiles, and therefore techniques for making effective use of energy generated by the vehicle that would simply be discharged conventionally are currently being developed. For example, a waste heat utilization device employing a Rankine cycle to convert heat discharged from the engine, such as the heat of cooling water and the heat of exhaust gas, into power for a power generator and so on is available. The Rankine cycle is constituted by a boiler that generates superheated steam by isobarically heating a liquid phase fluid serving as a working fluid of the Rankine cycle, an expander that obtains power by adiabatically expanding the superheated steam, a condenser that liquefies the steam expanded in the expander by isobarically cooling the expanded steam, and a pump that feeds the liquefied liquid phase fluid to the boiler.
Japanese Patent Application Publication No. 2010-65587 describes an example of this type of conventional Rankine cycle, in which output assistance is provided to an engine using energy from waste heat collected by an expander. A Rankine cycle typically improves in efficiency as pressure of the working fluid increases, and therefore the pressure of the working fluid flowing out of the boiler is controlled in the vicinity of a high pressure limit (an upper limit pressure) by adjusting the mass flow rate of the working fluid flowing through the boiler.
However, the ratio between the rotation speed of the engine and the amount of waste heat from the engine, or in other words the amount of heat recovered by the boiler, is not typically fixed, and therefore, when the amount of heat recovered by the boiler is excessively large relative to the mass flow rate of the working fluid flowing into the boiler, for example, such that the temperature of the working fluid flowing out of the boiler increases, the density of the working fluid must be reduced by reducing the mass flow rate of the working fluid flowing through the boiler, or the amount of heat exchange performed in the boiler must be limited, in order to keep the pressure of the working fluid at or below the upper limit pressure. As a result, the waste heat of the engine cannot be used effectively, and therefore the thermal efficiency of the Rankine cycle decreases.